A throat swab is taken from a patient with a respiratory infection. After 24 hours, 1- to 3-mm colonies develop, which are round, mucoid, nonpigmented, and dimpled in the middle. What is the most likely pathogen, and how could you confirm the diagnosis on the culture?
What will be an ideal response?
The colonies described are characteristic of Streptococcus pneumoniae. Diagnosis of this pathogen is confirmed if they are alpha-hemolytic on blood agar when grown aerobically and beta-hemolytic when grown anaerobically. Pneumococcal colonies can be differentiated from other colonies of alpha-hemolytic strains by adding a drop of bile to a colony. The bile triggers chemicals in S. pneumoniae that lyse the cells. The colony dissolves in a few minutes, confirming that they are pneumococcal colonies. Diagnosis can also be made on uncultured samples using the Quellung reaction in which antibodies specific for the capsule carbohydrates causes the capsule to swell.
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